1. Field of the Invention
The present invention relates to an ultrasound unit having a plurality of capacitive ultrasound transducer cells, and an ultrasound endoscope including the ultrasound unit.
2. Description of the Related Art
An ultrasound diagnostic method for performing diagnosis by applying ultrasound to a subject, and visualizing an internal state of a body from an echo signal has been popular. An ultrasound endoscope is one of ultrasound diagnostic apparatuses used for the ultrasound diagnostic method. In the ultrasound endoscope, an ultrasound unit is disposed at a distal end portion of an insertion section that is to be inserted into a body. The ultrasound unit has a function to convert an electrical signal to ultrasound and transmit the ultrasound into a body, and also to receive ultrasound reflected in the body and convert the ultrasound to an electrical signal.
In the ultrasound unit, a piezoelectric ultrasound transducer using a piezoelectric ceramic material (e.g., PZT: lead zirconate titanate), or a capacitive ultrasound transducer produced by using an MEMS technology (capacitive micro-machined ultrasonic transducer: c-MUT) is used as an ultrasound transducer.
In a c-MUT type cell, a bottom electrode and a top electrode that constitutes a membrane are arranged facing each other with a cavity therebetween. When a voltage is applied between the electrodes of the cell, the membrane is deformed by an electrostatic force to generate ultrasound. On the other hand, when reflected ultrasound (echoes) reflected from an object enters the cell, the membrane is deformed. Thus, ultrasound is received by measuring capacitance between the electrodes.
Here, increasing transmission sensitivity and increasing reception sensitivity of the cell contradict. Therefore, for example, Japanese Patent Application Laid-Open Publication No. 2005-510264 discloses an ultrasound unit having a transmit-only cell having a structure emphasizing transmission sensitivity, and a receive-only cell having a structure emphasizing reception sensitivity.